1. Field of Invention
This invention relates to display devices. This invention also relates to methods of making display devices.
2. Description of Related Art
Transmission-type, liquid-crystal display devices are currently the main stream of the liquid-crystal display devices. These devices require a back-light source on the back of the liquid crystal, the devices consume large amounts of power. In contrast, reflective-type, liquid-crystal display devices display an image by reflecting a light ray incident on the devices from an external light source, such as sunlight or room illumination, and thereby need no back-light source that consumes substantial power. Because of this feature, reflective-type, liquid-crystal displays operate on a substantially smaller power consumption level, and are thus expected to be incorporated in electronics used outdoors, such as portable telephones or portable information terminals.
In addition, reflective-type display devices have already found applications in projectors that use an external light source. A high-resolution feature is required for a display device that is incorporated in a projector. As the display device becomes high in resolution, the size of a unit pixel becomes small. For instance, a diagonally 0.9 inch VGA (video graphics array) display of 640×480 (307,200) pixels has a pixel size of about 28.6 μm. A SVGA (super video graphics array) display of 800×600 (480,000) pixels has a pixel size of 22.9 μm; an XGA (extended graphics array) display of 1024×768 (786,432) pixels has a pixel size of 17.8 μm; and a UXGA (ultra high extended graphics array) display of 1600×1200 (1920,000) pixels has a pixel size of 11.4 μm.
Reflective-type, liquid-crystal display devices are constructed by successively stacking a transparent electrode, a liquid crystal, an array of pixel electrodes that reflect incident light from an external light source, and an array of control elements for controlling the alignment of the liquid crystal on the respective pixel electrodes. An example of the construction of reflective-type, liquid-crystal display device is described in U.S. Pat. No. 6,049,132, which was co-invented by this inventor, hereby incorporated by reference in its entirety. Further, an example of the construction of a projector using reflective-type display devices is described in U.S. Pat. No. 5,757,054, hereby incorporated by reference in its entirety.
A disadvantage of the conventional reflective-type display device is discussed with reference to FIGS. 11-13.
FIG. 11 is a circuit diagram showing a portion of a conventional liquid-crystal display device including four rows by four columns of control elements. Each of the control elements 14 includes a switching circuit 28 including a PMOS transistor 24 and an NMOS transistor 26 connected in parallel. The pixel electrode, the liquid crystal, and the transparent electrode are connected between one terminal of the switching circuit 28 and the ground.
The gate of the PMOS transistor 24 and the gate of the NMOS transistor 26 are respectively connected to a pair of row drive lines 16 in each switching circuit 28 in the direction of the row, i.e., the horizontal direction in FIG. 11, in the control element 14. Each switching circuit 28 in the direction of the column, i.e., in the vertical direction, is configured with one terminal thereof connected to the respective pixel electrodes and the other terminal commonly connected to a column drive line 18.
FIG. 12 shows the layout of the display device of FIG. 11. FIG. 13A and FIG. 13B respectively show the layout and the circuit diagram of a single pixel. The shape of the unit pixel is typically square, although rectangular-shaped pixels with various aspect ratios may be utilized depending on the intended use. In any case, an aspect ratio of the pixel, or a ratio between the horizontal and vertical pitches of the pixel electrodes, may not be arbitrary determined, but is determined by the intended use. To configure the unit pixel in a square, or a fixed aspect ratio rectangular shape, the area of each control element should also be designed in the square shape, or in the fixed aspect ratio rectangular shape.
In many cases, a liquid-crystal display device requires higher operation voltage than the operation voltage of standard logic devices to drive a liquid crystal. Therefore, it is difficult to miniaturize the control element beneath the layer of the pixel electrode. Further, the square, or the fixed aspect ratio rectangular shape, of the pixel adds to the difficulty of miniaturizing the pixel. That is, when a practically available transistor design rule is implemented, a layout with a minimum space may require a rectangular area that does not match the shape of the pixel electrode. This presents difficulty in design of the control elements with a minimum area.